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From: Robert A Duff <bobduff@shell01.TheWorld.com>
Newsgroups: comp.lang.ada
Subject: Re: Ada 95 constructors on limited types
Date: Sat, 05 Jan 2008 09:59:07 -0500
Organization: The World Public Access UNIX, Brookline, MA
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Date: 2008-01-05T09:59:07-05:00
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Ludovic Brenta <ludovic@ludovic-brenta.org> writes:

> Brian May writes:
>>>>>>> "Jeffrey" == Jeffrey R Carter <spam.jrcarter.not@acm.nospam.org> writes:
>>
>>     Jeffrey> shaunpatterson@gmail.com wrote:
>>     >> 
>>     >> package Parent is
>>     >> 
>>     >> type Class is abstract tagged limited null record;
>>     >> type Class_Access is access all Class'Class;
>>     >> 
>>     >> end Parent;
>>
>>     Jeffrey> There's your problem. Don't use public access types.
>>
>> Why not?
>
> Because they automatically make the package impure for no good reason,
> thereby preventing use of the package in pure or preelabotated
> packages.  Moral: Declare your access types only if and where you need
> them.

Jeff was complaining only about PUBLIC access types.
I don't entirely agree with him -- at least, I'd put
it less strongly.

But anyway, the purity issue applies to access types
whether they are public or not.  I agree that it's
good to make things Pure when possible.

Note that the rule was relaxed in Ada 2005 -- you can say:

    type T is access Blah;
    for T'Storage_Size use 0;

and still have pragma Pure.  You can also have access-to-constant
and access-to-subprogram types.

- Bob